The present invention relates to a process and an apparatus for the reduction of switch-on surges in the operation of an inductive rotary current load provided with a magnetizable core having three windings arranged selectively in different connection groups on a rotary alternating current network, with at least a temporary phase operation of the voltages between the lines being provided.
In the operation of inductive loads, a magnetic remanence or a remanent induction remains in the magnetizable core after a switching off of a current. This magnetic remanence or remanent induction can be differently poled depending on the clarity of the alternating voltage at the switching-off point and may also have different values.
When switching on inductive loads, their remanence is usually unknown, and a high surge in a switch-on current occurs especially, e.g., in transformers having high induction and few air gaps, if the switch-on time point is unfavorable to the present remanence relative to the sequence of the alternating voltage. This switch-on surge can, under circumstances, amount to fifty times the nominal current and can lead to a tripping of safety components. These high currents are the result of the magnetization being driven far into saturation as a consequence.
For this reason, the induction must be reduced to such an degree that the switch-on current does not exceed a predetermined limit even in the most unfavorable event. This, however, requires considerably more iron in the core, e.g., of a transformer, while maintaining the same nominal power, making the core correspondingly larger and more expensive, which has negative effects, particularly, in high-power transformers.
Switching one of the three voltages between the lines to the respective winding, and thereby enlarging the voltage-halfwave phase angle is already known from PCT/DE91/00216. Simultaneously with a measurement of the reactive current, a magnetizing current ensues. This measurement of the reactive current monitors the operational state at which the magnetization of the core reaches saturation.
This switching-on process has proven itself, but it demands measurement of the current and requires devices for evaluating the measurement and for the respective control of the correcting components. Moreover, a relatively complex and expensive follow-up control must be provided, because after flowing current through one of the windings by switching on one voltage between the lines, the other voltages have to be switched on following a predetermined period of delay in correct sequence until magnetic saturation is reached. The circuit is, therefore, very complex.
Attempts have already been made to conduct this switch-on process without measuring the current and with an effective voltage that has been reduced to a fixed value. In this case, the core is premagnetized by applying one of the three voltages between the lines. However, it turns out that an effective reduction of the switch-on current surge is greatly dependent on the loading and on the quality of the inductive load so that changing and adapting the switch-on process has to be carried out in dependence on the connected, inductive load and, e.g., in the case of a rotary current transformer also in dependence on its secondary load. However, this is complicated and prevents universal use.